FIG. 6 shows an example of a conventional thin planar speaker. This speaker includes a yoke 50; a plurality of bar-shaped magnets 52 which are arranged in parallel on the yoke 50; a diaphragm 54 which is arranged in parallel with the pole faces of the bar-shaped magnets 52; and a plurality of coils 56 which are arranged on the diaphragm 54 at positions facing the bar-shaped magnets 52 such that current flows in a direction perpendicular to a magnetic field generated from the bar-shaped magnets 52. When alternating current is caused to flow through each of the coil 56, according to Fleming's left hand rule, force is generated between the coil 56 and the magnetic field, with the result that the diaphragm 54 is vibrated in a direction perpendicular to the surface of the diaphragm. That is, by causing alternating current to flow through the coil 56 in accordance with predetermined electrical signals, the electrical signals are converted into sound signals.
However, the above-described planar speaker involves problems, including generation of noise resulting from twisting of the diaphragm 54 due to force along the surface of diaphragm 54, which force occurs by the effect of the magnetic field perpendicular to the surface of the diaphragm 54, since, for example, the coils 56 facing the bar-shaped magnets 52 have an elongated rectangular shape, and most portions of the coils 56 are located in regions facing the pole faces of the bar-shaped magnets 52.
In an attempt to improve the planar speaker of FIG. 6 involving the above-mentioned problem, a planar speaker having a configuration shown in FIG. 7 has been proposed. In the planar speaker having this configuration, a plurality of magnets 62 are arranged on a yoke 60 in parallel with a diaphragm 64 such that the pole faces of adjacent magnets differ from each other. Furthermore, a plurality of spiral coils 66 are arranged on a surface of the diaphragm 64 on the side facing the magnets 62, such that the innermost circumference of each of the spiral coils is positioned in the vicinity of a portion of the diaphragm, the portion corresponding to the outer periphery of each of the pole faces.
With the configuration of the planar speaker described above, force which the coils 66 receive from a magnetic field perpendicular to the diaphragm 64 is reduced, and generation of noise is suppressed. In addition, the portions of the coil 66 that are perpendicular to a magnetic field in parallel with the diaphragm are increased in area, whereby sound conversion efficiency is enhanced, and sound pressure which is a yardstick for sound conversion efficiency is increased.
However, the planar speaker shown in FIG. 7 involves problems of noise and sound pressure, etc. An improved technique for the planar speaker involving such problems has been proposed in Patent document 1. This document discloses a planar speaker having voice coils formed by employing a wiring technique disclosed by Patent document 2.
The wiring technique disclosed in Patent document 2 is the technique in which a wiring head which is provided so as to be movable relative to the surface of a sheet-like base having an adhesive layer on at least one surface thereof (hereinafter referred to as “adhesive sheet) is intermittently brought into point contact with the surface of the adhesive sheet, while a wire conductor is fed from the wiring head, thereby attaching the wire conductor onto the surface of the adhesive sheet in a sequential manner.
With use of the above-described wiring technique, spiral voice coils are provided on the adhesive layer of the adhesive sheet, and then another sheet having the same adhesive layer as that of the adhesive sheet is bonded to the adhesive sheet with the voice coils sandwiched therebetween such that the adhesive layers thereof adhere to each other, thereby faulting a diaphragm. The planar speaker described in Patent document 1 is composed of the thus-formed diaphragm and magnets provided at a position facing the voice coils.    Patent document 1: Japanese Patent Application Publication No. 2003-284187    Patent document 2: Japanese Patent Application Publication No. 2001-126942.